U.S. Secretary of State Rex Tillerson steps out of his plane upon arrival in Moscow's Vnukovo airport, Russia, April 11, 2017.

LONDON - U.S. Secretary of State Rex Tillerson is set to meet Wednesday with Russian Foreign Minister Sergei Lavrov with a focus on Syria, Ukraine and U.S.-Russia relations.

Tillerson arrived in Moscow with less ammunition than Washington and London had hoped he would have in his bid to convince Russia to abandon Syrian President Bashar al Assad.

But he does have a tough ultimatum in hand, following reports quoting unidentified senior U.S. officials as saying that Russia had prior knowledge of the attack that killed scores of people including women and children.

Watch: Tillerson Has Tough Talk for Russia's Role in Syria

G7 ministers meeting Tuesday in the Italian city of Lucca failed to agree on targeted sanctions against the Russian and Syrian military, arguing that an investigation would first have to confirm who in Syria used chemical weapons against civilians in the country last week.

“We cannot let this happen again,” Tillerson told reporters before flying to Moscow.  “We want to relieve the suffering of the Syrian people.  Russia can be a part of that future and play an important role,” he said.  “Or Russia can maintain its alliance with this group, which we believe is not going to serve Russia's interests longer term.”

The chemical attack prompted a world outcry and a U.S. missile attack that marked a turning point in the Trump administration's approach to the seven-year-old conflict.

Victims of the chemical weapons attack lie on the

U.S. President Donald Trump and British Prime Minister Theresa May had agreed to press Russia to distance itself from Assad following the chemical attack by imposing targeted sanctions, but Germany and Italy, both leading G-7 nations, disagreed.

Russian President Vladimir Putin “must not be pushed into a corner,” said Italian Foreign Minister Angelino Alfano said Tuesday.

For his part, Putin called Tuesday for a U.N. probe of last week's attack. Without elaborating, he also said Russia has received intelligence about planned “provocations” using chemical weapons that would put the blame on the Syrian government.

The G7 ministers' decision in Italy now means the prospect of sanctions is dim. The process of launching an investigation would be long and complex, requiring a U.N. resolution and an agreement by the Assad government for weapons inspectors to access sites in territory under Assad's control before establishing who was responsible and whether there was Russian complicity.

Syria's President Bashar al-Assad is interviewed F
Syria's President Bashar al-Assad is interviewed Feb. 10, 2017, in Syria. Thousands of Afghans from Iran are involved in the Syrian conflict, many sent against their will, Human Rights Watch reports.

As the ground rapidly shifted regarding the U.S. approach to Syria, Tillerson made it clear that Washington hopes Assad will not be part of Syria's future.  He told the foreign ministers in Lucca U.S. missile strikes were necessary as a matter of U.S. national security, and indicated the Trump administration may not be done with Assad.

“We do not want the regime's uncontrolled stockpile of chemical weapons to fall into the hands of ISIS or other terrorist groups who could, and want, to attack the United States or our allies.  Nor can we accept the normalization of the use of chemical weapons by other actors or countries in Syria or elsewhere,” Tillerson said.